I'd suggest +1 fireball damage per level, but that would get out of control too easily.

Not really. That's a 25% damage increase, whereas the -1 MP per fireball works out to a 20% more damage for the same mana. Overall this is a worse class feature than his current mana discount since it would only apply to fireballs.

Your suppositions rely on the idea that 1) piety gain for JJ between Wizards and Paladins would be equal. Given that BURNDAYRAZ is not a spell that grants piety for JJ, and BYSSEPS and GETINDARE are (as well as the spellcasting applications of WEYTWUT and APHEELSIK, although a Wizard could likely make equal use of those), in practise a Paladin will gain more piety than a Wizard on top of having cheaper boons. 2) You regard monotheism in MA and EM as a curse - I disagree. In fact, quite often one of the last things I do before ending a level is an Entanglement; I've been known to convert out of other religions to pull that off. It helps that EM is very hard to piss off, too. She's also got benefits all game; Vine Form early game to get HP up, with mid-to-end game Clearances for mana chaining or Entanglement for eating popcorn/final blow on a boss. Greenblood's a little rubbish unless you stack it 5+ times against multiple resistant targets, but that's her only ability even close to useless. I think you underestimate her (probably because her "punishment" is pathetic, making her an easy desecrate target.) Mystera monotheism makes less sense - after you've converted every glyph to Refreshment, there's no point to sticking with her. Of course she's also the crappier deity to and I'd only end up taking her if JJ wasn't around.

Totally unrelated sidenote, but I miss JJ's Polymorph. That was a fun ability, even though I understand they had to axe it to prevent people from turning the Indomitable into, say, Super Meat Man and making a joke out of him.

1) piety gain for JJ between Wizards and Paladins would be equal. Given that BURNDAYRAZ is not a spell that grants piety for JJ, and BYSSEPS and GETINDARE are

That's stretching things, especially since the wizard is more than capable of using these glyphs in a piety farming context. You don't actually need to use the effects, just trigger them. In any case, the Paladin would need to be two levels of boost mana ahead of the wizard to have a discernable advantage, and this isn't going to add up to that much piety.

Furthermore, this case is the most extreme in the Paladin's favour. If you add in other factors, consider other deities or combos, the Paladin really has no advantage over the wizard.

2) You regard monotheism in MA and EM as a curse - I disagree. In fact, quite often one of the last things I do before ending a level is an Entanglement; I've been known to convert out of other religions to pull that off

Yes, and Paladin does not have the option of doing so. He does not have the option of converting into Earthmother after leveraging another deity, nor converting out after grabbing the boons he wants from her. With plantation on the menu, piety literally grows on trees and the sky's the limit in terms of conversion antics.

It helps that EM is very hard to piss off, too.

This is not a point in the Paladin's favour, since being able to brazenly piss off his chosen deity is one of his core advantages. It's not worth a whole lot in the case of EM. He's basically just got the 20% discount on one hand and the inability to convert on the other. On a deity with such massive piety generation potential, I'd give up the discount in a heartbeat to convert.

I think you underestimate her (probably because her "punishment" is pathetic, making her an easy desecrate target.

I'm not saying EM is bad; I'm saying she works best with the option to convert. She's basically the only deity with the piety generation to grab everything on the menu and still convert out with 100 piety. For a conversion-heavy approach, Earthmother is great. And yes, desecrating her once you're done with worship is yet another bonus.